Hidden Faults Reported Off S. Calif.

Sunday, October 1st 2000, 12:00 am

By: News On 6

LOS ANGELES (AP) â€” Two hidden faults capable of unleashing a magnitude-7.6 earthquake lie off the coast of heavily populated Los Angeles, Orange and San Diego counties, researchers reported Sunday.

Though there's potential for catastrophe, the chances are slim. In the worst-case scenarios detailed in the study, the biggest quakes occur once every 2,100 years on one of the faults â€” the Thirtymile Bank fault â€” and every 8,800 years on the other â€” the Oceanside fault.

It's possible the faults release their energy in smaller but more frequent spurts, the researchers reported in the October edition of the journal Geology.

``Because this is new and we can't access it easily, we don't have the knowledge base yet to decide whether it is going to rupture in small pieces or in one single event,'' said study author John H. Shaw of Harvard University.

``The critical issue for hazard assessment is really just defining the size of these faults,'' he added. ``The size obviously dictates the potential earthquake magnitude.''

A 7.6-magnitude quake would likely cause widespread damage and injuries. The 6.7-magnitude Northridge quake in 1994 killed 72 people and caused an estimated $35 billion in damage in Los Angeles.

The Thirtymile Bank fault runs south from Santa Catalina Island, and the Oceanside fault slices south from Laguna Beach in Orange County. Both extend south to San Diego and possibly beyond the U.S.-Mexico border.

Both faults are the same type that unleashed the Northridge and 1971 Sylmar quakes. Called blind thrust faults, they are not clearly visible on the surface, whether on land or on the sea floor, and are usually detected when they produce quakes.

``This is the first concrete evidence that we have large thrust faults in the offshore region here,'' said Tom Henyey, director of the Southern California Earthquake Center. ``It is a significant finding, if in fact it is the case.''

Other offshore faults, such as the Newport-Inglewood and Rose Canyon faults are strike-slip faults, where one side slides horizontally past the other.

Thrust faults, where one side moves over the other as if on a ramp, could pose greater threats because their quakes tend to have higher vertical acceleration.

``This tends to be very destabilizing for many types of structures, including high-rise buildings and other things,'' Shaw said.

And below the ocean, vertical movement might produce tsunamis that wash over coastal areas, Shaw said. The researchers did not analyze tsunami potential of the faults.

The researchers from Harvard and the University of Colorado at Boulder used data collected by oil companies exploring for petroleum. The seismic reflection profiles, created by measuring aspects of sound from small explosions, are sonograms of the Earth.

``In this picture, you see the layers of rock, and in some cases you can see the faults directly,'' Shaw said.

The faults have little historical record. Just one magnitude-5.3 quake off Oceanside on July 13, 1986, likely originated on the Thirtymile Bank fault, the researchers said.

``We have such a short history with thrust faults, we just don't know over the long term how these things go,'' Henyey said. ``It's possible you'll get small earthquakes now and then on these things and then all of sudden the whole thing will just go in one shot.''